Well, although Chelsea is a more solid side than Real Madrid, comparing the Barça x Chelsea and the Madrid x Barça game is a mistake.
First of all, different competitions, championship and 2 leg cup competition.
Chelsea played at Camp Nou with one simple task in mind: not losing. They had 4 defenders, and 5 midfielders that were defending more than anything else.
Madrid had a do or die game, they needed to win no matter what, they only had 2 midfielders with defensive duties.
Again, to me Chelsea is a more solid team than Madrid, but many of you seem too confident on their defensive abilities. Surely they can't play with 10 men behind the ball at home, and Barça will have more space to work with.